Improving ergonomics of extend

[Boscop]

It would be nice to have an extend method on f32: fn extend(self, y: f32) -> Vec2.

Also the Vec2::extend method could take something that impls Into<ExtendRemaining2>, and the f32::extend method could take something that impls Into<ExtendRemaining3>.
ExtendRemainingN is a trait that types implement that can fill up the remaining N "slots".
E.g. f32 impls ExtendRemaining2 because it can be appended when there are 2 free "slots".
Vec2 also impls ExtendRemaining2 but not Vec3.
E.g. you want to be able to do p.extend(q) when both are Vec2 to create one Vec4.
Or 1f32.extend(p) when p is a Vec2 (to create a Vec3) or when p is a Vec3 (to create a Vec4).

Related to #109, #110 and #112

[bitshifter]

Can you provide a code example of how you would like to use extend on f32? This would require adding a trait and implementation for primitive types, which is not something glam currently does or has any other need for.

For extending to other types, I would be more inclined to add more From conversions, e.g. there are currently:

• impl From<(Vec2, f32)> for Vec3
• impl From<(Vec3, f32)> for Vec4

I would be OK with continuing this pattern and adding:

• impl From<(Vec2, f32, f32)> for Vec4
• impl From<(Vec2, Vec2)> for Vec4

Which wouldn't require adding additional traits to glam.

[bitshifter]

Added all the above From impls in my comment, going to close this as won't fix for now. I don't want to add these additional traits to glam.